This application is a continuation of application Ser. No. 830,863 filed on Feb. 19, 1986, now abandoned.
1. Field of the invention
The present invention relates to a liquid crystal color display device defined by a plurality of picture elements arranged in a plurality of rows in a matrix and, more particularly, to an arrangement of switching elements coupled to each picture elements.
2. Description of the Background Art
The liquid crystal color display device generally comprises a plurality of picture elements arranged in a dot matrix. Each picture element is provided with a coloring means for producing one particular color of a set of additive primary colors. Each picture element is applied with a controlled voltage to produce its assigned color at a brightness determined by the video signal. Thus, by combining the colors of the neighboring picture elements in the same principle as used in a color cathode ray tube, an image defined by any desired colors including half tone colors and natural colors can be displayed.
The prior art liquid crystal color display device has a plurality of picture elements aligned at a predetermined pitch in two orthogonal directions, e.g., horizontal and vertical directions on a transparent substrate with a plurality of straight signal electrodes extending vertically between picture elements for sending color signals to picture elements in columns and a plurality of straight scan electrodes extending horizontally between picture elements for identifying the row which is to be scanned. If the picture elements are provided at a high density, a high quality image having colors as natural as that of the original can be reproduced. However, in the case where the number of picture elements to be contained is limited, the quality of the reproduced image differs greatly depending on the manner in which the picture elements of additive primary colors are aligned. Some alignments show very poor spatial resolution in the vertical direction, resulting in undesirable moire pattern revealing on the display screen.
In consideration of the above disadvantages, an improved liquid crystal display device employing a delta alignment of the picture elements is disclosed in U.S. patent application Ser. No. 722,591 (European patent application No. 85 104 516.1) assigned to the same assignee as the present case. The delta alignment according to U.S. patent application Ser. No. 722,591 (European patent application No. 85 104 516.1) is shown in FIG. 1. Picture elements 104, actually the electrodes for such picture elements, are aligned in rows, each row extending horizontally. In each row, picture elements are aligned with a predetermined pitch P so as to provide a predetermined space S1 between two adjacent picture elements. The rows extend parallel to each other with a predetermined space (the widest is shown as S2) between each two adjacent rows. As shown in FIG. 1, the picture elements 104 in the odd rows are in off set relationship with the picture elements 104 in the even rows by a 0.5 pitch spacing of the pitch in which the picture elements are aligned horizontally. Thus, the three additive primary colors, e.g, red (R), blue (B) and green (G) are disposed closely adjacent to each other in a shape similar to a character "delta".
To drive each picture element, a plurality of straight scan electrodes 101 are provided, each extending along the upper side of each row of picture elements. Also provided are a plurality of signal electrodes 102 each defined by vertically extending portions 102v and horizontally extending portions 102h appearing alternately. As illustrated in FIG. 1, the vertically extending portions 102v are located in a space between two picture elements aligned horizontally, and the horizontally extending portions 102h are located in a space between two picture elements in two adjacent rows. Each scan electrode 101 has branch electrodes 101b each extending in a space between two picture elements aligned horizontally. The branch electrode 101b partly overlaps with the vertically extending portion 102v and partly overlaps with a drain electrode 103 which further overlaps partly with an electrode defining the picture electrode, so as to form a thin film transistor at a space between two picture elements aligned horizontally. The thin film transistors are formed on the right and left sides alternately of the signal electrode 102 so that each signal electrode carries one particular color signal.
The delta alignment of the picture elements as described above has a problem in that relatively large spaces are occupied by electrodes 101 and 103, resulting in less space remaining for placing the picture elements, which inevitably results in reduction of brightness of the image on the screen. More specifically, a thin film transistor requires a relatively wide space between picture elements 104; and scan electrode 101 and horizontally extending portion 102h, which are positioned side-by-side, occupy a relatively large space between picture elements in two rows. The reduction of available space for the picture elements becomes greater as the number of picture element increases.
To solve the above problem, one may conclude that the width of scan electrode 101 and horizontally extending portion 102h might be narrowed. However, this results in the increase of the ohmic resistance of the electrodes, thereby undesirably weakening the signal.
Also, it is possible to place horizontally extending portion 102h over scan electrode 101 to increase the space for the picture element. However, the overlap structure of the electrodes 102h and 101 results in the increase of the capacitance between the two electrodes 102h and 101, which adversely affects the signal transmitted through signal electrode 102.